1. Field of the Invention
The present invention relates to a reflow soldering apparatus for soldering electronic parts to a substrate and, more particularly, to a reflow soldering apparatus providing uniform distributions of temperature and velocity of heating gas and enabling a variation of both the angle at which the hot gas impinges upon the substrate and the length over which the hot gas is applied to the substrate.
Hitherto, reflow soldering methods and apparatus have been proposed in which the local portion where the soldering is to be conducted is heated by a single nozzle or a plurality of nozzles. Such methods and apparatus are disclosed, for example, in Japanese Patent Laid-Open Publication Nos. 53-75148, 54-40255, 59-113689, 61-208291 and 61-255762. On the other hand, Japanese Patent Laid- Open Publication No. 57-132395 discloses a method which can eliminate any lack of uniformity in the soldering attributable to variation in the heat capacities of the electronic parts. A method disclosed in Japanese Utility Model Laid-Open Publication No. 59-61567 employs an arch- type tunnel in which pre-heating is conducted by applying hot air. This method produces an appreciable effect in improving the quality. However, since air having a small heat transfer coefficient is used as the heat medium, it is necessary that the air velocity is increased to a level which would not cause the electronic parts to be blown away. To this end, it is necessary to employ an air blowing means. The aforementioned Japanese Utility Model Laid-Open Publication No. 59-61567 and Japanese Patent Laid-Open Publication Nos. 61-289697, 63-1177960, 63-180368, 63-278668 and 63-296295 disclose the use of a blower for blowing the heating air. It is, however, difficult to blow pre-heating or heating air with high degrees of uniformity of temperature and velocity. In Japanese Patent Laid-Open Publication No. 61-289697, the hot air is directed in parallel with the direction of conveyance of the parts to be heated, while Japanese Patent Laid-Open Publication No. 63-177960 discloses the provision of a flow settling plate disposed at the outlet side of a blower so as to realize a uniform distribution of the hot air over the entire width of the parts to be treated. Japanese Patent Laid-Open publication Nos. 67-84870 and 63-180368 employ flat plates with ports for blowing hot air.
The above-noted, however, still require improvements in the following respects.
Hot air which is circulated through a pre-heating chamber and the interior of the reflow chamber is spread towards the objects to be heated at a spreading ratio which is 5.0 or greater. Thus, the air is not spread uniformly and, hence, the flow velocity distribution of the air, as well as the heat transfer, is not uniform. Consequently, a non-uniform temperature distribution is developed over the objects to be heated.
In order to attain uniform distribution of the air velocity both in the direction of conveyance of the object to be heated and in the breadthwise direction, it is necessary to employ a flow resistance plate such as a flow settling plate or a perforated flat plate with air blowing ports. The air blowing means therefore is required to have a capacity large enough to overcome the increase in the flow resistance posed by such a flow settling plate or perforated plate. This leads to an elevated levels of the power consumption and noise.
The flux components contained in the solder applied to the substrate is evaporated during melting of the solder so as to be mixed in the heating air. The flux components suspended by the air is circulated through the system so as to be deposited to the flow resistance member. The deposition of the flux components not only increases the flow resistance across the flow resistance member but also impairs uniformity of hot air distribution because of non-uniform deposition. In addition, a frequent cleaning of the flow resistance member is required thereby increasing the maintenance cost.